In metal forming operations, the presence of a metal working lubricant is a necessity. Without a suitable lubricant, the friction between the die and the workpiece is so great as to cause galling, scoring, and even tearing of metal. These problems are exacerbated in operations involving the formation of deep sections, for example two-piece metal beverage cans, vehicle oil pans, and particularly products of thick sections such as spark plug bases.
In the past, articles of relatively shallow section could be coated with a film of lubricating oil or a coating of a metallic soap. However, as the use of fewer drawing stages and stronger workpiece alloys pushed the processing envelope, such crude lubricants rapidly became obsolete. Further, the use of lubricating liquids and soft soap films is not conducive to the manufacturing environment, the former because of their inherent messiness, and the latter due to the softness and hygroscopicity of the films produced. Stearate and other fatty acid salts have been found to be problematic with respect to washing operations, causing plugging of drains.
Such lubricants are also incapable of being used in many modem metal forming operations where surface temperatures of the dies and metal workpieces may often exceed 500.degree. C. and may occasionally rise to temperatures of c.a. 1000.degree. C. or higher due both to friction generated between the die and workpiece as well as the heat generated internally in the workpiece due to plastic flow of metal. At these temperatures and at the high pressures associated with metal forming, even common "high pressure lubricants" are completely ineffective.
In addition to being lubricious under extreme operating conditions, a suitable metal forming lubricant must also possess other characteristics in order that it may be successfully used in a commercial setting. For example, the lubricant must not build up on the die, otherwise "break through" or striations may be formed. In some cases, the lubricant may form a residue of sufficient size such that the fully formed workpiece contains hollows corresponding to the built up residue, and thus produces a part which is not the mirror image of the die. This is particularly true with respect to solid inorganic lubricants such as graphite, vermiculite, molybdenum disulfide, and the like.
Furthermore, in most cases, it is desirable to coat the workpiece with lubricant remote from the metal forming operation. For example, metal blanks may be coated, dried, and shipped to the metal forming plant by a supplier. It is thus necessary that the lubricant coating be solid, non-tacky, and non-dusting. It is further necessary that the lubricant coating be sufficiently hard to resist damage during handling and shipping. Particularly for ferrous metal parts, the coating should be relatively non-hydroscopic and should not contain salts which promote rusting or corrosion. Examples of the latter are borates and nitrites, particularly the former.
Preparing lubricants with these often conflicting goals has proven difficult.
In U.S. Pat. No. 4,752,405 is disclosed a lubricant containing a metal soap, in this case an alkali metal salt of a C.sub.12-30 fatty acid; a polyoxyethylene glycol having a molecular weight in the range of 1500 Da to 8000 Da; an acrylic film forming polymer; and a variety of surface active agents to promote complete mixing of the ingredients. However, while dried films of the lubricant composition exhibited improved hardness, the films were still relatively hygroscopic, absorbing only slightly less water than films containing metal soaps and metal borates, such as the films exemplified by U.S. Pat. No. 3,725,274. The water absorption is believed due to the use of polyoxyethylene glycols which themselves are considerably hydrophilic. The metal soaps and polyoxyethylene glycols, while being excellent low temperature, low pressure lubricants, lose their lubricity at higher temperatures and pressures, and are thus not suited for many modern deep drawing operations.
In U.S. Pat. No. 4,654,155 is disclosed a water-emulsifiable metal rolling lubricant containing a complex organic phosphate ester, an amine, a polyoxyalkylated oil, one or more polyoxyalkylene glycol or polyol esters, and a non-esterified polyoxyalkylene glycol. The composition was found to be highly lubricious by the three ball test. However, the composition is only suitable for operations where liquid coatings may be tolerated, such as metal rolling operations. Moreover, none of the ingredients is a high temperature, high pressure lubricant.
In U.S. Pat. No. 4,474,669 is disclosed a lubricant composition containing molybdenum disulfide, an acrylic ester, acrylic acid polymer, and a polyethylene wax in aqueous dispersion. The coating may be applied to a metal surface such as a beverage can blank and dried. Cans formed by deep drawing lubricant-coated steel compared favorably to cans formed from tin-plated steel in which the tin plating is naturally lubricious. However, the composition of U.S. Pat. No. 4,474,669 contains molybdenum disulfide (moly). Moly is widely known as a high pressure metal lubricant. However, it is very expensive and today is environmentally suspect. Thus, it must be recovered and disposed of properly or recycled, adding further to manufacturing cost. Moly also tends to leave deposits on the die.
A variety of compositions have been marketed which employ combinations of polyethylene wax with acrylic film forming polymers with and without other ingredients such as organic phosphate esters. Similar compositions containing polyvinylchloride polymers instead of polyethylene are also known, such as those disclosed in U.S. Pat. No. 3,725,274. Such compositions have been found suitable for modest drawing operations not involving either high temperature or exceptionally high pressure. Under the latter conditions, the films lose their lubricity, and galling, tearing, and other effects occur with regularity. Attempts to extend the range of such compositions by adding high temperature resistant lubricious polymers such as polytetrafluoroethylene (PTFE, Teflon.RTM.), polyvinylidene fluoride, and the like have not been successful. While lubricity is in some cases satisfactory, the fluorinated polymers have been found to leave a residue which requires frequent cleaning and reconditioning of the die.
It would be desirable to provide to the metal forming industry a metal lubricant which may be used as a dry, durable coating; which is useful even at exceptionally high pressures and temperatures; which is environmentally friendly; which is substantially free of hygroscopic borates or metal soaps; and which leaves very little residue on metal dies. It would be further desirable to provide such a lubricant in liquid form for those applications not requiring a previously applied coating. It would yet be further desirable to provide metal lubricants which leave a conductive residue on the formed parts.